Cellulose ester film has been employed as a photographic negative film support, and in polarizing plates as a film which protects polarizers employed in liquid crystal displays, due to its high transparency, low birefringence, and ease of adhesion to polarizers.
In recent years, the production amount of liquid crystal displays has markedly increased due to the thin depth and light weight, and the demand is increasing. Further, television sets, which employ a liquid crystal display, exhibit features such as thinness and light weight. Thereby, large-sized television sets, production of which was not possible by employing Braun tubes, have been produced. Along with that trend, demand for polarizers and polarizer protecting films has been increasing.
Heretofore, these cellulose ester films have been produced mainly employing a solution-casting method. The solution-casting method, as descried herein, refers to a film forming method in which a solution prepared by dissolving cellulose ester in solvents is cast to form film and solvents are evaporated and dried to produce film. The film which is cast employing the solution-casting method exhibits high flatness, whereby by employing the resulting film, it is possible to produce uniform and high image quality liquid crystal displays.
However, an inherent problem of the solution-casting method is the necessity of a large volume of organic solvents followed by a high environment load. The cellulose ester film is cast employing halogen based solvents which result in a high environment load, due to its solubility characteristics. Consequently, it has particularly demanded to reduce the amount of used solvents, whereby it has been difficult to increase the production of cellulose ester film employing the solution-casting method.
Accordingly, in recent years, experiments have been conducted in which cellulose ester is subjected to melt-casting for the use of silver salt photography (Patent Document 1) and as a polarizer protective film (Patent Document 2). However, cellulose ester is a polymer which exhibits a very high viscosity when melted and also exhibits a very high glass transition point. As a result, when cellulose ester is melted, extruded from a die and cast onto a cooling drum or belt, it is difficult to achieve leveling, and after extrusion, solidification occurs in a relatively short time, whereby a major problem has been that flatness of the resulting film is inferior to that of the a solution-casting film.
In order to lower the melt viscosity and glass transition point of organic polymers such as cellulose ester, it is known that addition of plasticizers is effective.
In above Patent Documents 1 and 2, employed are phosphoric acid plasticizers such as triphenyl phosphate or phenylenebisdiphenyl phosphate. However, the result of investigations conducted by the inventors of the present invention has clarified that in these phosphoric acid plasticizers, phosphoric acid esters undergo decomposition due to moisture sorption or heating, resulting in generation of phosphoric acid, whereby problems occur in which generated phosphoric acid degrades cellulose ester and a film is stained.
In the solution-casting, known as plasticizers, other than phosphoric acid esters, which are employed in cellulose ester, are ethylene glycol based plasticizers or polyhydric alcohol based esters which are esters of trihydric or higher alcohol with carboxylic acids. For example, Patent Document 3 discloses glycerin-carboxylic acid based esters, Patent Document 4 discloses diglycerin-carboxylic acid esters, Patent Document 5 discloses pentaerythritol or dipentaerythritol based esters, and Patent Document 6 discloses sugar alcohol based esters such as sorbitol. Plasticizers composed of polyhydric alcohol-carboxylic acid exhibit relatively high chemical stability, and even when hydrolyzed, do not generate strong acids which degrade cellulose ester, whereby they are preferable plasticizers for casting of cellulose ester. However, most of them are alkyl ester based, resulting in insufficient effects to lower water vapor permeability. Further, Patent Document 7 discloses polyhydric alcohol-aromatic carboxylic acid and polyhydric alcohol-cycloalkylcarboxylic acid based esters. However, it has been found that such compounds having a ring structure result in insufficient effects to lower viscosity as a plasticizer during melt-casting of cellulose ester, whereby problems occur in which it is not possible to prepare cellulose ester films which exhibit flatness.
Further, there was a problem of bleeding out of a plasticizer, i.e., deposition or evaporation of a plasticizer getting out of the film.
Further, in any of Patent Documents 3-7, described are neither specific synthesis examples nor specific compounds of aromatic esters having a substituent, and no effects of having the substituent are also described. In addition, no description is made for the application of these plasticizers to a melt-cast cellulose ester film.
(Patent Document 1) Japanese Patent Publication Open to Public Inspection (under PCT Application) No. 6-501040
(Patent Document 2) Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 2000-352620
(Patent Document 3) JP-A No. 11-246704
(Patent Document 4) JP-A No. 2000-63560
(Patent Document 5) JP-A No. 11-124445
(Patent Document 6) JP-A No. 2001-247717
(Patent Document 7) JP-A No. 2003-12823